In another question here (sanity of a plastic glass!) the term "plastic glass" is being used which sounds somewhat odd to me, but has not been brought into question by any respondents.

Maybe it is has to do with the fact that my mother tongue is German where a "Glass" is always made from glass, while a plastic cup would be be "Plastikbecher". (which would translate into plastic cup.)

Up to this moment I would have agreed entirely with you and said that in Britain a glass is a glass and a plastic beaker is a plastic beaker. But we are currently staying in our daughter's house and I notice that she has some drinking receptacles made of plastic, that are on stems, a bit like the ones they give you on airlines for wine. I am not sure what I would call them other than 'plastic glasses'.
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WS2Nov 12 '13 at 8:39

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Ha! I was thinking the exact same thing. An oxymoron expression you might call it. I think what the OP is asking whether we can define a glass object as being plastic. In this case, it is the vessel, for want of an alternative word, that is made of plastic. But whoever says a plastic drinking vessel?
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Mari-Lou ANov 12 '13 at 8:50

The problem is that plastic can be used to make mugs, beakers and other types of cup, (which were originally made of wood, metal, earthenware or whatever), but also to make tumblers, flutes, and other types of glass (which were, when named, only made out of glass). ? A plastic is too vague to catch on, IMHO.
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TimLymingtonNov 12 '13 at 18:48

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Interestingly I have asked for a "glass glass" in a pub I went to because I realized the "glasses" they were using were plastic. The bartender understood what I wanted.
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John SmithNov 13 '13 at 0:46

4 Answers
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Only a slight addition to what's been said before, but I think it adds something.

A plastic glass (in the UK) would normally be used to refer to a plastic substitute for a drinking vessel usually made of glass (e.g. for picnics/camping or at concerts), and would normally be transparent (though as with glass glasses, many styles exist, translucent coloured wine "glass"es being quite common in camping shops).

A plastic cup may be provided for water, terrible coffee, etc. and is often white, though sometimes (for water) is transparent. IME this is always a disposable cup. A camping coffee cup made of plastic would be a plastic mug (and shaped like a normal mug), while a vessel for soft drinks might (in the UK, but not the US) be a beaker (the qualifier plastic isn't really needed there), with childish implications.

Finally for another meaning of glasses, "plastic glasses" can be used to refer to spectacles with plastic lenses, or with plastic frames.

I've never heard the word 'beaker' outside of a chemistry lab or The Muppet Show.
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Amish ProgrammerNov 12 '13 at 13:54

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I'd never thought about it but apparently (wikipedia) it is a UK term.
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Chris HNov 12 '13 at 15:36

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@BrianDHall, Thanks, I've updated to reflect UK-specific usage. perhaps any contributors from the rest of the commonwealth might like to chip in. Tumbler is fairly common in the UK at least for a rather squat glass.
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Chris HNov 12 '13 at 17:19

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Just to add another difference - a plastic cup is usually considered flimsy and easily compressible to make it easily disposable. The vessel being clear in that case would not make it a plastic glass since they usually mimic the properties (thickness and rigidity) of a vessel made of glass.
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James SnellNov 12 '13 at 23:15

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@JamesSnell, good point in the general case, except in the rock concert example above, the plastic pint "glasses" (not "cups" IME) would be flimsy, compressible and disposable.
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Chris HNov 13 '13 at 8:23

Sure. Why not? A chamois cloth needn't be made of chamois leather. A wood golf club needn't be made of wood. In Britain, a copper (2p or 5p coin) isn't made of copper. And my glasses have plastic lenses.

If you want to be pedantic about it, you can call it a clear, plastic beaker. But in that case, make sure the polymer the beaker is made from is a thermoplastic polymer, rather than a thermoset...

@Pitarou I am enjoying this. Maybe I chose the wrong word, since translucency, transparency and even opacity are all properties of various types of glass. Glass containers in the context of "wine glasses" are available in designs incorporating these properties. What I meant to say is that a plastic glass would have to mimic the design and optical properties of an ordinary glass of the same function. To add fuel to the fire, modern camping "glassware" includes containers shaped like wine glasses but made from stainless steel.
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NamSandStormNov 12 '13 at 11:43

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It was an open secret that the company had used a paid volunteer to test the plastic glasses. Although they were made using liquid gas technology and were an original copy that looked almost exactly like a more expensive brand, the volunteer thought that they were pretty ugly and that it would be simply impossible for the general public to accept them. On hearing this feedback, the company board was clearly confused and there was a deafening silence. This was a minor crisis and the only choice was to drop the product line. (Much Ado About English. Nicholas Brealey Publishing, 2006)
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wimNov 12 '13 at 17:28

In summary, as long as the "alternative" plastic glass resembles the "original" glass in function and appearance, it is a valid and acceptable word?
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NamSandStormNov 12 '13 at 11:58

Correct. You will see from the tableware retailers that you can get wine glasses, tumblers, pint glasses, half pint glasses, shot glasses, etc. - all made of plastic.
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SteNov 12 '13 at 12:01

The same goes for camping retailers and the stainless steel equivalents they sell, but are they an acceptable authority on the use of the English language?
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NamSandStormNov 12 '13 at 12:05

I wouldn't say that they are an acceptable authority by a long stretch of the imagination but I was attempting to illustrate that the term "plastic glasses" is ubiquitous, certainly in the UK.
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SteNov 12 '13 at 12:07

And for further information, I have never known a steel container be referred to as a "glass". I think you'd be headed for "goblet" territory there!
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SteNov 12 '13 at 12:07

a hard, brittle, noncrystalline, more or less transparent substance produced by fusion, usually consisting of mutually dissolved silica and silicates that also contain soda and lime, as in the ordinary variety used for windows and bottles.

any artificial or natural substance having similar properties and composition, as fused borax, obsidian, or the like.

something made of such a substance, as a windowpane.

a tumbler or other comparatively tall, handleless drinking container.

glasses, Also called eyeglasses. a device to compensate for defective vision or to protect the eyes from light, dust, and the like, consisting usually of two glass or plastic lenses set in a frame that includes a nosepiece for resting on the bridge of the nose and two sidepieces extending over or around the ears

It's worth noting that the merriam-webster.com & the oxforddictionaries.com sites reserve "glass" for a drinking vessel made from glass. But this has definitely become a popular (although lazy) saying for any type of relatively tall, handleless drinking vessel.

You might get downvotes for spelling Merrian-Webster and the Oxford Dictionary in lower-case letters. ELU are a very pedantic lot.
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Mari-Lou ANov 12 '13 at 9:08

Only Oxford Dictionary—the other is a URL, and it would be incorrect to capitalise it. ;-)
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Janus Bahs JacquetNov 12 '13 at 9:23

Thanks @Mari-LouA and Janus Bahs Jacquet. I revised me answer to be technically and stylistically correct. Unfortunately, I didn't reference a physical copy of the Oxford Dictionary.
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THEAONov 12 '13 at 9:29

Good answers here, but between the Oxford, Merrian-Webster and dictionary.com the authors seem to have different views on this.My 1992 edition of Webster's Dictionary and Thesaurus also is specific that the container should be made from glass. On the other hand it is 21 years old...
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NamSandStormNov 12 '13 at 11:03